1. Field of the Invention
The invention relates generally to bipolar radio frequency (RF) integrated circuits having high frequency vertical NPN transistors and low frequency lateral PNP transistors and more particularly to an RF integrated circuit using a lateral PNP transistor as a variable resistor or as an RF signal carrying component in a variable attenuator or RF switch.
2. Description of the Prior Art
Discrete radio frequency (RF) circuits commonly use PIN diodes as variable resistors for signal path switching and controlled signal attenuation. A PIN diode, illustrated in FIG. 1b, has a P+ region having a high density of positive carriers, an N+ region having a high density of negative carriers, and an I region of intrinsic material having a low intrinsic carrier density disposed between the P+ and N+ regions. An electrical charge is stored in the I-region that is proportional the volume of the I-region and the carrier density. The separation of the N+ and P+ regions by the I region causes the PIN diode to have a favorable characteristic for RF circuits of having a relatively low capacitance. In operation an external bias voltage and current is applied to the PIN diode through electrical connections to the P+ and N+ regions. When a positive voltage (forward bias) is applied, a forward bias current having an exponential relationship to the voltage passes through the I region. The stored charge due to the carriers in the I-region is proportional to this bias current. Importantly, the PIN diode has an effective RF resistance that decreases as the stored charge increases, with the result that the PIN diode becomes a variable RF resistor having a resistance that depends upon the forward bias current. Typically, a relatively low bias current of a few milliamps is required to store enough charge to reach an RF resistance of a few ohms. For frequencies higher than a few tens of megahertz, a relatively large RF signal applied across the forward biased PIN diode does not significantly add or subtract to the stored charge in the I region at the peaks of the signal, and therefore distortion of the RF signal is low. When a negative voltage potential (reverse bias) is applied from the P+ region to the N+ region a very small reverse current passes through the I region causing the PIN diode to have a high RF resistance.
Existing silicon bipolar RF integrated circuits use a set of standard components that are characterized for the processes of designing and manufacturing the integrated circuit. These components typically include fixed resistors, capacitors, PN diodes, high frequency NPN transistors that are diffused vertically into a certain area of a silicon substrate, and low frequency PNP transistors using separate horizontally spaced (lateral) diffused areas of the substrate. The lateral PNP transistors have a much lower frequency response than the NPN transistors so they are not generally used as RF signal carrying components in RF circuits within the integrated circuit but are nevertheless included for convenience in designing low frequency circuitry. A great deal of engineering and testing goes into characterizing the standard components and designing the manufacturing and design processes so that the components are as predictable as possible. It should be appreciated that deviating from the set of standard components is prohibitively expensive in both design and manufacturing and may yield unpredictable results. Therefore when cost is any consideration, it is important for the RF engineer to adapt his custom design to the standard components rather than vice versa. Because PIN diodes are not included in the set of standard components, RF engineers have used PN diodes as variable resistance elements in RF integrated circuits. However, as compared to a PIN diode a PN diode, illustrated in FIG. 1a, exhibits less desirable characteristics of having greater distortion and a higher capacitance and of requiring a greater bias current for operation.
There is a need for an RF integrated circuit having a variable RF resistor having a low capacitance, low distortion, and low operating current using the set of standard components that are characterized for the processes of designing and manufacturing the integrated circuit.